Overview no. 5

Microstructure Evolution of Ruthenium During Vacuum Hot Pressing : Obtaining fine-grained metal with mechanical properties close to electron-beam melted ruthenium

Ruthenium tablets with mean grain size of ~4‐5 μm were prepared by vacuum hot pressing (VHP), and tablets with maximum density of 12.2 g cm‐3 were obtained with sintering time of 2 h. X-ray diffraction (XRD) revealed that there was a texture change with sintering time. The microstructure of the ruthenium tablets was observed by electron backscatter diffraction (EBSD) and field emission scanning electron microscopy (FSEM). The microstructure evolution of ruthenium with sintering time is discussed.

Modelling the formation and self-healing of creep damage in iron-based alloys

A self-consistent model is applied to predict the creep cavity growth and strain rates in metals from the perspective of self-healing. In this model, the creep cavity growth rate is intricately linked to the strain rate. The self-healing process causes precipitates to grow inside creep cavities. Due to the Kirkendall effect, a diffusional flux of vacancies is induced in the direction away from the creep cavity during this selective self-healing precipitation. This process impedes the creep cavity growth. The critical stress for self-healing can be derived, and an analysis is made of the efficiency of self-healing elements in binary Fe-Cu, Fe-Au, Fe-Mo, and Fe-W alloys. Fe-Au is found to be the most efficient self-healing alloy. Fe-Mo and Fe-W alloys provide good alternatives that have the potential to be employed at high temperatures.

Design and Synthesis of Metals (Tungsten) with Structural Hierarchy for Very High Temperatures

The strength of metals is limited by the weakest link in their microstructures. At high temperature, the weakest link is usually a grain boundary. Mobile dislocations represent another type of weak link. The strength-limitation by this two-level hierarchy of crystal defects can be minimized through microstructure design. Although the strengthening mechanisms differ for grain boundaries and dislocations, it is possible to design microstructural architectures that strengthen both. The design must take the stress state into consideration that the component will encounter during its later use. Processing strategies can then be devised for the synthesis of such components. It is imperative to stabilize the designed microstructure at high temperature because this will determine how well and how long the material will be able to perform. Design, synthesis and stabilization are discussed in the present paper. The lamp filament wire technology is the basis for a new method that enables the synthesis and stabilization of artificial microstructures in high-temperature materials.

Mechanical Stability of Passivation Films Deposited on Aluminium Substrates

The damage of films of two different passivation materials deposited on aluminium substrates was investigated. These systems were subjected to uniaxial stretch in an in situ tensile testing device adapted to a scanning electron microscope. Successive stages of crack development were observed: the failure is first in the film, then at the interface. The presence of a thermally grown aluminium oxide interlayer delays the decohesion process. The substrate surface roughness weakens the systems. The evolution of the crack density with the longitudinal strain was analysed and then fitted with a model based on a Weibull distribution function.